The business aircraft marketplace requires that business aircraft cruise long distances at near supersonic speeds. Such performance requires careful integration of the engine-propulsion system with external airframe aerodynamics.
Extensive investigation of various aircraft configurations indicates that significant benefits may be achieved by utilizing the forward swept wing configuration. In an aft-swept wing configuration, spanwise air flow normally thickens the boundary layer at the wingtips. In a forward swept wing configuration, air flow tends to separate first at the inboard section of the wing while good flow conditions are maintained at the wingtips. Thus, higher aerodynamic efficiency is exhibited with forward swept wings than with aft swept wings. Such flow conditions result in stall characteristics which render the ailerons effective at high angles of attack after most of the wing has stalled, making the aircraft controllable at relatively high lift coefficients.
However, since a forward swept wing tends to stall first on the inboard wing sections rather than on the outboard sections, air flow control over the fuselage must be carefully controlled to insure stability in low speed flight, at high angles of attack, and at high subsonic speeds.
In order to assess the effect of a forward swept wing on fuselage boundary layer air entering the engine air inlet, analyses were performed with forward and aft swept wings at the same root position. It was determined that an aft swept wing has little influence on forward fuselage pressure. The boundary layer thickens from about 1 inch at the forward end of the cylindrical portion of the fuselage to about 2 inches at an intermediate portion of the fuselage adjacent the air inlet.
However, the aforesaid adverse pressure gradient along the side of the fuselage is nearly eliminated with the forward swept wing. Body surface streamlines sweep up into a low pressure region induced ahead of and above the wing. Due to the intrustion of the forward swept wing into the flow field forward of the inlet and tapered fuselage section, the principles of area ruling cause the flow to fill the converging taper and flow smoothly into the inlet. As a result, the thickness of the boundary layer adjacent an engine air inlet located at this point is reduced. This is particularly important in a high angle of attack situation where the boundary layer sweeps up from the under side of the fuselage and converges along the top of the fuselage. Thus, fuselage shape is important in avoiding flow separation upstream of the engine air inlet.